import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution4 {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return bool布尔型
     */
    public boolean isValidBST (TreeNode root) {
        // write code here
        if(root == null) {
            return true;
        }
        if(root.left == null && root.right == null) {
            return true;
        }
        if(root.left != null && root.left.val >= root.val || root.left != null && root.left.right != null && root.left.right.val >= root.val) {
            return false;
        }
        if(root.right != null && root.right.val <= root.val || root.right != null && root.right.left != null && root.right.left.val <= root.val) {
            return false;
        }
        return isValidBST(root.left) && isValidBST(root.right);
    }
}